In the manufacturing process of semiconductor devices and the like, in order to prevent contaminants that lead to short circuits from adhering to very fine electrical circuits on the wafers, the wafers are conveyed while housed in a hermetically sealed state inside a clean container 2 as shown in FIG. 20, to be processed in a clean room. In the drawing, reference numeral 2a indicates a front door of the clean container 2 and 4 indicates wafers, the wafers 4 being housed on a plurality of shelves inside the clean container 2.
Incidentally, when wafers are processed in the clean room, in the prior art the entire clean container is brought to the clean room and, after the front door is opened by hand, the wafers are extracted using a robot or the like prepared separately, and during that extraction, it is necessary to recognize the number of wafers and the positions of shelves where the wafers exist (hereafter referred to as “mapping”), in order to control the robot or the like.
FIGS. 21 and 22 show a wafer treatment device of the prior art; FIG. 21 is a partial cutaway side view of the device, and FIG. 22 is a plan view of the device. In the drawings, reference numeral 1 indicates a stage, and at one side of the stage 1 (the left side in the drawing), a stand 3 on which the clean container 2 is mounted and a wafer mapping device 5 elevating for detecting the wafers 4 inside the clean container 2 on the stand 3 from the outside of the clean container are provided. Also in the drawing, reference numeral 6 is a wafer conveying robot provided on the other side (the right side in FIGS. 21 and 22) of the stage 1, 7 is a control device for the wafer treatment device, and 8 is a wafer processing device for processing wafers 4 extracted from the clean container 2 by the wafer conveying robot 6.
The wafer mapping device 5 herein supports an abbreviated U-shaped arm frame 5a, arranged so that its upper end portions are respectively positioned in the forward and rearward directions (right and left directions in FIGS. 21 and 22) in the center of the left and right directions of the clean container 2 (up and down directions in FIG. 22) with a screw 5c rotated by a motor 5b, provides a light emitting device 5d and light receiving device 5e in the upper end portions of the abbreviated U-shaped arm frame 5a, and, by rotating the screw 5c by means of the motor 5b, elevates the light emitting device 5d and light receiving device 5e on the upper end portions of the arm frame 5a screwed onto the screw 5c between the lower end and upper end (in the lower end position in the drawing) of the clean container 2.
The clean container 2 is normally constructed to house 10 to 25 wafers, and because it is not known on which shelf a wafer 4 is housed, the wafer treatment device firstly performs operations to recode which shelf a wafer 4 is on inside the clean container 2, by means of the wafer mapping device 5. That is, when the clean container 2 is mounted on the stand 3, a control device 7 of the wafer treatment device operates the motor 5b, elevating the arm frame 5a, to which the light emitting device 5d and the light receiving device 5e are attached, from the lowest position to the highest position at a preset speed. During this process, the control device 7 recognizes the positions of wafers 4 by a combination of a difference in the output signal of the light receiving device 5e, which receives a light beam from the light emitting device 5d, between when a wafer 4 intervenes between the light emitting device 5d and the light receiving device 5e and when it does not, and the amount of movement of the arm frame 5a. Then when the arm frame 5a rises to the highest position, the consul device 7 conveys the wafers 4 on specified shelves among the recognized wafers 4 to the wafer processing device 8 by means of the wafer conveying robot 6, and stores, on speed shelves inside the clean container 2, wafers 4 in the wafer processing device 8 whose processing has been completed, by means of the wafer conveying robot 6.
However, with this method, the disadvantages of it being unfavorable to bring the clean container 2 whose exterior is contaminated into the clean room in terms of maintaining cleanliness levels inside the clean room, and that the light beam does not correctly reach the light receiving device 5e if the clean container 2 is light shielded or if it is deformed even if transparent, etc. have occurred.
For this reason, recently, a semiconductor manufacturing process has been proposed in which a high cleanliness area called a mini-environment that separates a given limited section where the wafer processing device 8 is located is provided, and a load port, which is a device for extracting and inserting wafers, is installed at the interface between the high cleanliness area and a middle or a low cleanliness area wherein the clean container 2 presents, the clean container used therein being called a FOUP (Front Opening Unified Pod), standardized under SEMI (Semiconductor Equipment and Materials Intentional) together with the load port, so that semiconductor device manufacturer around the world are in the process of employing this system.
As a wafer mapping device in this mini-environment system, there is, for example, that which is provided in the wafer treatment device load port 12 disclosed in the patent application opened No. 2000-133697 filed by the applicant of the present invention, as shown in FIGS. 23 and 24. FIG. 23 is a partial cutaway side view of the wafer treatment device, and FIG. 24 is a cross sectional view of the wafer treatment device seen from above along the line A—A in FIG. 23. The load port 12 therein is constructed so as to make the port door 13 tightly contact to the front door 2a of the closed type clean container (FOUP) 2 that houses wafers 4 on a plurality of interior shelves and elevate them by means of a ball screw 16b rotatably driven by a servo motor 16a, a pair of sensor portions 14 are attached at both side ends of the upper end surface of the port door 13 so that they are each rotatable 90° horizontally on an axis at one end thereof, the pair of sensor portions 14 opening 90° when the port door 13 and the front door 2a of the clean container 2 contacts tight and descend, a light emitting device 14a and light receiving device 14b provided respectively in the sensor portions 14 enter the front end portion of the clean container 2, and face each other at separated positions from the port door 13, and a control device 7 recognizes wafers 4 based on the output electrical signal of the light receiving device 14b that receives light from the light emitting device 5d. 
According to the wafer mapping device of the patent application opened No. 2000-133697, because the light emitting device 14a and light receiving device 14b provided in the sensor portions 14 enter into the front end portion of the clean container 2 and face each other at positions separated from the port door 13 to recognize wafers, if the clean container 2 is light shielded or transparent but deformed, the light from the light emitting device 14a usually arrives correctly at the light receiving device 14b and reliably recognizes wafers 4, but upon further study of this wafer mapping device, the inventor of the present application noticed that there is a point at which improvement is preferred, having found that because the pair of sensors portions 14 each rotate 90° horizontally, there is a possibility that minute contaminants (trash) may be generated and may intrude into the clean container (FOUP) 2, and that for the light from the light emitting device 14a of one sensor portion 14 to be accurately incident on the light receiving device 14b of the other sensor portion 14, the 90° horizontal rotation stopping positions of the sensor portions 14 must both be extremely accurate, so that there is the possibility that after operating for a long time they do not accurately stop at the 90° position.